There are a number of applications in which solid particles are extracted from a liquid. Particles to be examined may be captured by a filter or may be passed by the filter while larger, undesired particles are blocked. Some applications involve separating parasite larvae and eggs from feces placed in a specimen vial to which a diluent liquid is added. One apparatus utilizes a funnel carrying a screen through which the feces-liquid mixture is poured from the specimen vial into a centrifuge tube. The screen catches large debris such as undigested vegetable matter and passes larvae, eggs and diluent liquid. The tube is centrifuged and the parasites are removed for examination.
In many fecal examination procedures, such as one using the above apparatus, the feces-liquid mixture is not maintained throughout the procedure in a closed system. In such cases the mixture is subject to spillage which invalidates the sample.
Another apparatus utilizes a double-ended cap which threadably engages both a specimen vial and a centrifuge tube. The cap carries a screen including a coaxial tube to provide for the passage of gas to equalize pressure between the tube and the vial. During use, feces and diluent liquid are added to the specimen vial and the cap with inverted centrifuge tube is screwed over its opening. The assembled apparatus is then inverted end-for-end to pass liquid and parasites into the centrifuge tube while blocking undesired debris.
The coaxial tube is required to prevent a vacuum from forming inside the specimen vial and to allow displacement of gas from the centrifuge tube. However, the single tube is easily blocked by debris. The apparatus often must be agitated frequently to clear the tube. Further, the tube may be crimped during manufacture or may become separated from the cap. When the tube is removed, little or no liquid passes through the screen.
Even when the coaxial tube operates as designed, the liquid is transferred at a low flow rate. Additional pressure equalization tubes would decrease available screen area. Also, the double-ended threaded cap is expensive to manufacture and to assemble with the coaxial tube.